Optical memory for storing data

ABSTRACT

The present invention concerns optical memories. Such an optical memory (30) comprises a data line (31) which is optically coupled via a directional waveguide coupler (33) to a circular memory loop (32). In addition, it comprises a pump line (35), which is employed in order to couple a refresh signal into the loop (32). As in case of the data line, the pump line is coupled via a directional waveguide coupler (34) to the loop (32). The counter propagating refresh light pulse provides for an amplification of the light pulse circulating in the memory loop being doped with Erbium ions. The length L of the memory loop (32) is chosen such that the circulation frequency of the light pulse in said loop (32) is equal to the clock frequency of the optical memory (30). The memory provides for individual manipulation of each stored optical bit.

TECHNICAL FIELD

The present invention concerns an apparatus for storing light pulses ina small waveguide memory loop.

BACKGROUND OF THE INVENTION

Photon storage, in analogy to electron storage such as the well knowncapacitor memory cell, is very difficult because of the physical problemof containment of a photon or photon stream. Up to now, most approachesfor storing photons are either based on the effect of opticalbistability or on the utilization of long fiber loops.

Devices making use of the optical bistability are usuallytechnologically complex and expensive. Much of the early work on opticalbistability was motivated by the idea that optics could avoid some ofthe intrinsic speed limitations of electronic storage systems. Thedevices based on the effect of optical bistability rely on electro-opticconversion and hence an electron is stored in this case. Atwo-dimensional access or emitting function can be implemented withthese kind of devices.

Fiber loops exist in the form of sequential storage pipes, where aphoton stream, e.g. a sequence of light pulses, flows or recirculates ina first-in first-out (FIFO) fashion. These fiber loops are used asoptical delay lines, but do not allow a random-access to the lightpulses circulating in such a loop. A typical photon memory device basedon a recirculating fiber loop is described in "Programmable PhotonicFiber Loop Memory", A. Dickson et al., Proceedings of the 16thAustralian Conference on Optical Future Technology, p. 274-277, 1991. Atypical recirculating fiber loop memory 10 is illustrated in FIG. 1.This memory 10 consists of a directional coupler 14 for coupling lightinto it. This memory 10 comprises an Erbium doped fiber amplifier (EFA)11 with a 70 meters long Er⁺ -fiber, a fiber loop 12 (fiber coil) whichis 160 meters long, a semiconductor laser amplifier (SLA) 13 operatingas optical switch, and two optical isolators 15, each being depicted asbox with a black arrow. These kind of devices are bulky and expensive.

For future functions in optical processing and data communication it isdesirable to have a photon storage device in much analogy to aconventional electronic cache memory, i.e. there is a great demand foran optical memory being as easy to use as their electronic counterparts.These kind of devices will most likely be used in many fields ofphotonics, including telecommunications, packet switching, and opticalcomputing. Usage of an optical memory is foreseen in bit-selectivemanipulation at high data rates (e.g. 10 Gb/sec), like processing,temporal storage and recognition. It is very important for such adevice, that it is capable of storing and manipulating optical bitsindividually. Ideally it should be integrable to optically performelemental logical functions. Any conversion of photons into electronsand vice versa should be avoided in order to increase efficiency and tominimize access time to data stored.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an apparatus foroptically storing light pulses carrying information.

It is another object of the present invention to provide an apparatusfor optically storing light pulses carrying information such that eachlight pulse stored is individually addressable.

The above objects have been accomplished by provision of memory cellscomprising a data line, a directional waveguide coupler which can beswitched from a `bar-state` to a `cross-state`, and a circular waveguidememory loop for storing a light pulse circulating therein, the data linebeing optically coupled via the directional waveguide coupler to theloop such that a light pulse on the data line is coupled into saidmemory loop if the directional waveguide coupler is in the`cross-state`, or passes the directional waveguide coupler--withoutbeing coupled into said memory loop--if it is in the `bar-state`. Thelength L of said memory loop is chosen such that the circulationfrequency of said light pulse in the memory loop is equal to the clockfrequency of said optical memory.

DESCRIPTION OF THE DRAWINGS AND NOTATIONS USED

The invention is described in detail below with reference to thefollowing schematic drawings:

FIG. 1 shows a conventional recirculating fiber loop used for the delayand storage of a sequence of bits (background art).

FIG. 2 shows an optical random-access memory according to the presentinvention.

FIG. 3A is a timing diagram of the clock signal of an optical memory inaccordance with the present invention.

FIG. 3B is a timing diagram of the data signals on the data line of anoptical memory in accordance with the present invention.

FIG. 3C is a timing diagram showing the status of the directionalwaveguide coupler of an optical memory in accordance with the presentinvention.

FIG. 3D is a timing diagram showing the intensity of a light pulsecirculating in the waveguide loop of an optical memory in accordancewith the present invention.

FIG. 3E is a timing diagram showing the data output signals on the dataline of an optical memory in accordance with the present invention.

FIG. 3F is a timing diagram showing suitable pump pulses for refresh ofthe light pulse circulating in the waveguide loop of an optical memoryin accordance with the present invention.

FIG. 4A shows an optical memory cell, according to the presentinvention, which has a separate pump line with active directionalwaveguide coupler.

FIG.4B is a schematic, cross-sectional view of the memory cell shown inFIG. 4A.

FIG. 4C is a sketch of the directional coupler of the memory cell shownin FIG. 4A.

FIG. 5A shows an optical memory cell, according to the presentinvention, which has a separate pump line coupled via a passivedirectional waveguide coupler to the loop.

FIG. 5B shows an optical memory cell, according to the presentinvention, which has a separate pump line coupled via a Y-branch to theloop.

FIG. 5C shows an optical memory cell, according to the presentinvention, which comprises an semiconductor amplifying region.

FIG. 6 shows a set-up which allows to erase data bits. FIG. 7 shows a4-bit wide random-access optical memory, according to the presentinvention, which has four individual input and output lines.

FIG. 8 shows a 4-bit wide random-access optical memory, according to thepresent invention, which has four individual input and output lines andfour individual, separate pump lines.

FIG. 9 shows a 4-bit wide random-access optical memory, according to thepresent invention, which has one common data line linking the memoryloops in a serial manner.

FIG. 10 shows a 4-bit wide random-access optical memory, according tothe present invention, which has four individual input and output linesand one common pump line.

FIG. 11 shows a 4-bit wide random-access optical memory, according tothe present invention, which has one common data line and one commonpump line.

FIG. 12 shows a 4-bit wide FIFO type of optical memory.

FIG. 13 is a schematic, cross-sectional view of two memory loops stackedon top of each other.

FIG. 14 is a schematic, cross-sectional view of of a stripe-loadedwaveguide structure.

GENERAL DESCRIPTION

For the sake of simplicity, it is assumed in the following, that a lightpulse represents-one bit of information. This is not necessarily so, butfor most applications this approach is reasonable.

The present invention will be described in more detail in connectionwith an optical memory consisting of a single bit memory cell 20 only,as illustrated in FIG. 2. This memory cell 20 at least comprises acircular memory loop 22, a directional waveguide coupler 23, and a dataline 21. These basic elements are arranged such that a light pulse canbe fed via said data line 21 and said directional waveguide coupler 23into said memory loop 22. The directional waveguide coupler 23 can beelectrically or optically addressed in order to switch from a so-called`bar-state` to a `cross-state`. If the directional waveguide coupler 23is in the `bar-state`, the light pulse passes the loop 22 without beingcoupled into it. If the coupler 23 is in the `cross-state`, the lightpulse is coupled into the loop 22. Switching between these two statesfacilitates selective storage of light pulses in said memory loop 22. Alight pulse being coupled into this loop 22 circulates therein. Theduration of this circulation is limited due to waveguide propagationlosses in the loop. These losses are caused mainly by the waveguidematerial as such and by bending loses depending on the particulargeometry of the loop.

Details on directional waveguide couplers are for example described inthe book "An Introduction to Photonic Switching Fabrics", H. S. Hinton,Plenum Press, New York and London, 1993. Further details are disclosedin "Optical Switching Expands Communications-Network Capacity", W. H.Nelson et al., Laser Focus World, June 1994, pp. S17-S20. For sake ofconvenience, directional waveguide couplers are depicted in the Figuresas a simple box carrying a symbol meant to represent two waveguides witha narrow section. In this narrow section, the energy is coupled from thefirst waveguide into the second one, depending on the state (`cross`- or`bar-state`) of the coupler. A classification of such couplers is givenin the following.

Basic categories of waveguide couplers:

1. Interference based couplers, utilizing the optical interferencecaused by a phase difference in two arms of the waveguide coupler. Thephase difference can for example be introduced by electro-, acousto-, orthermo-optic effects.

2. Internal reflection optical couplers, relying on a (total) internalreflection principle inside the waveguide structure. The internalreflection zone acts as a mirror and can for example be introduced as arefractive index change into the waveguide by carrier injection viaelectrodes. Details are for example given in "InGaAsP/InP opticalSwitches Using Carrier Induced Refractive Index Change", K. Ishida etal., Applied Physics Letters, Vol. 50, No. 3, pp. 141-142, 1987.

3. Gain/Absorption couplers, in the shape of a waveguide tree orY-branch with a 3 dB splitting ratio and optical gain sections in bothlegs. Details are for example given in "Integrated Lossless InP/InGaAsP1 to 4 Optical Switch", Davies et al., Electronics Letters, Vol. 28, No.16, pp. 1521-1522, 1992. The switch function uses two effects: (i) inthe switched leg, optical gain is provided to raise the opticalintensity to the required (original) intensity and (ii) in thenon-switched leg, the optical signal undergoes a well definedattenuation via the absorption loss and hence a certain extinction ratiowith respect to the switched channel (i) is obtained.

4. Modal evolution couplers, where the propagation or attenuation of adesired optical mode is given by the matching of the modes in therespective waveguide branches. Such matching is accomplished by thewaveguide design and can for example be influenced by the electro-opticeffect using electrodes. Details are given in "Digital Optical Switch",Y. Silberberg et al., Applied Physics Letters, Vol. 51, No. 16, pp.1230-1232, 1987.

5. All-optical couplers: in this category, non-linear optical effectslike the refractive-index changes are induced by strong opticalintensities. Examples are the optical Kerr effect, Sagnac-typeinterferometer couplers like the nonlinear optical loop mirror (NOLM),and four-wave mixing.

Such directional waveguide couplers, which are either electrically oroptically addressable as described above, and clocked at the appropriaterate, are well suited for use in the systems herein described.

In order to allow random-access to a bit stored by means of a lightpulse circulating in such a memory loop, its position within the loopmust be predictable, and the circulation frequency of the light pulsehas to be equal to the clock frequency which is defined by theenvironment in which said memory is used. This facilitates random-accessto the data stored. The design of the waveguide loop has to be such thatthe circulation frequency matches the clock frequency defined by theunit which requests retrieval of data and which needs these data forfurther processing. The basic principle of the present invention isdescribed in the following in connection with FIGS. 3A-3F. In FIG. 3A,the system clock, i.e. the clock signal applied to the present opticalmemory, is illustrated. A clock cycle (timing clock period) is denotedby τ. Two light pulses 24 and 25, FIG. 3B, are fed via the data line 21to the optical memory. The first pulse 24 is coupled into the loop 22since the directional waveguide coupler 23 is in the `cross-state`, seeFIG. 3C, whereas the second pulse 25 is bypassed (coupler in`bar-state`). The light pulse being coupled into the loop 23 nowcirculates in it, as indicated in FIG. 3D, an passes the directionalwaveguide coupler each time when a clock signal is applied. The length Lof the loop 22 is in the present example chosen such that the pulsecirculation time in the loop is equal to τ, i.e. L=τc/n, where c is thefree-space speed of light, and n is the refractive index of thewaveguide material in said circular memory loop 22.

the beginning of each clock cycle, the pulse in the loop passes thedirectional waveguide coupler 23. If no pump pulse is applied to theloop, the signal is attenuated, i.e. the amplitude decreases with time,as illustrated in FIG. 3D by fully drawn boxes. With adequate pump pulsethe signal strength is held constant, as indicated by the dashed boxesin lo FIG. 3D. The light pulse 25 has not been coupled into the loop andpasses the coupler 23 as shown in FIG. 3E. The light pulse circulatingin the loop 22 is read out if the coupler is switched from `bar-state`into `cross-state`, as indicated in FIG. 3C. This pulse then appears onthe data line 21 (see pulse 26 in FIG. 3E). Appropriate pump pulsesignals 27 are shown in FIG. 3F. These pump signals, if properlysynchronized with respect to the stored (circulating) pulse, wouldrestore the original signal strength in the loop.

At a given data rate b, the length l of an individual light pulse inspace is given by l=c/nb, where c is the free-space speed of light, andn is the refractive index of the waveguide material in said circularmemory loop 22. For a data rate b=10 Gb/s and a waveguide made of acompound semiconductor material like GaAs or InP with n=3.2, the opticalpulse length within this waveguide loop is l=10 mm. The circumference Lof the memory loop 22 is in the present example chosen to match thislength l, i.e. the length L of the memory loop 22 is equal to theoptical pulse length l. The resulting loop radius r=(1/2 π)(c/nb) is 1.5mm. It has been found that this value for the radius r is still a factorof ten above the waveguide radius where the excess bending loss becomesimportant. Details on bends in optical ridge waveguides are given in thePh.D. thesis of E. C. M. Pennings with title "Bends in Optical RidgeWaveguides; Modelling and Experiments", Technical University of Delft,The Netherlands, 1990, p. 138. Excess bending losses seem to play animportant role if the radius is smaller than 0.15 mm.

In the cell design, the waveguide propagation loss of <0.7 dB/cm isherein used to a first approximation. The losses in the memory loop 22define the storage lifetime of a circulating pulse. If the signal tonoise (S/N) ratio is too small, the light pulse can not longer bedetected, i.e. retrieved, since due to the waveguide and propagationlosses, the optical bit intensity will be attenuated to a level belowdetection threshold.

In the above example, the directional waveguide coupler 23 is used forselectively coupling a light pulse into the memory loop 22. The samedirectional waveguide coupler 23 can be used for retrieving a bit whichis stored as circulating light pulse. If the directional waveguidecoupler 23 is switched from the `bar-state` to the `cross-state`, thelight pulse is coupled back into the data line 21. The data line forfeeding data to said coupler might be the same as the one used forretrieval of data, as illustrated in FIG. 2. Another embodiment isconceivable where there are two separate lines for storage andretrieval.

For some applications, it is sufficient to employ such a single bitmemory cell as illustrated in FIG. 2, in which the light pulse is onlystored for a fraction of a second. Depending on the design, material andradius of such a memory loop, the above mentioned waveguide loss allowsa number of approximately 10 loop circulations, which is equal to 10clock periods. Such single bit memory cells might be used forsynchronization purposes, or as buffers in optical switch fabrics.

In order to obtain an optical memory suited for long-term storage of abit, the light pulse circulating in the memory loop needs a refresh.This concept is similar to the one of conventional dynamic random-accessmemories which require a dynamic refresh in order to keep theinformation stored. If the light pulse is amplified, it can be storedfor an arbitrary time interval to allow functions like buffering oradditional processing. A dynamic random-access optical memory 30, inaccordance with the present invention, is described in connection withFIG. 4A. This optical memory 30, comprises a data line 31 which isoptically coupled via a directional waveguide coupler 33 to a circularmemory loop 32. In addition, it comprises a pump line 35, which isemployed in order to couple a refresh signal into said memory loop 32.As in case of the data line, the pump line 35 is coupled via adirectional waveguide coupler 34 to said loop 32. The counterpropagating refresh light pulse provides for an amplification of thelight pulse circulating in the memory loop, if said memory loop is dopedwith Erbium ions. These Erbium ions provide for optical gain whichcompensates the waveguide losses when an optical pump pulse is fed intothe loop, either in a direction against or with the signal stream(counter- or co-propagation). As a simplified scheme it is assumed thatthe Erbium ions in the host matrix of the loop form a three-level energysystem as described in "Design of 1480-nm diode-pumped Er³⁺ -dopedintegrated optical amplifiers", F. Horst et al., Optical and QuantumElectronics, Vol. 26, pp. S285-S299, 1994. The Erbium ions aretransferred from their ground state to the highest excited state by theincident pump light; from this level the ions relax to the slightlylower lasing level. From this lasing level the ions can return into theground state under emission of photons of equal energy as thepropagating signal and thus amplify the circulating flux at the signalwavelength (or energy). Under unpumped conditions, the optical bit thatis circulating in the loop will gradually be attenuated, whereas uponoptical pumping of the Erbium ions in the loop the intensity of thisoptical bit will be amplified and can be restored. For completeness ithas to be noted that the energy levels mentioned above are actuallysomewhat broadened energy bands and that the above mentioned transitionstake place between initial and final states within these bands: thisallows to use an energy window for lasing action instead of just asingle line. Further details on this physical effect are given in "TheGolden Age of Optical Fiber Amplifiers", E. Desurvire, Physics Today,American Institute of Physics, pp. 20-27, January 1994.

This mechanism of amplification is described here for the example ofErbium ions as amplifying medium as commonly used today in the 1,55 μmdata and telecommunication window. However, it should be noted that anyamplifying element either optically or electrically pumped has the sameeffect. In particular the refresh of data light pulses is not restrictedto a specific optical wavelength.

In the present example, this directional waveguide coupler 34 can beswitched from `bar-state` to `cross-state` such that one can selectwhether a refresh pulse is to be coupled into the loop 32 or not. If theinformation, i.e. the light pulse, stored in said loop is not longerneeded, the directional waveguide coupler 34 can be blocked such that norefresh signal goes into the loop. In this case no active reset isnecessary since the light pulse disappears within a fraction of asecond. Further, unpumped Er⁺ -ions act as absorbers for light in the1,55 μm range.

A cross-sectional view of the dynamic random-access optical memory 30 isshown in FIG. 4B. The waveguide layer has a height h and the waveguideshave a width w. The height is for example 3-5 μm and the width between 5and 10 μm. The directional waveguide coupler 33 comprises a narrowsection where the data line 31 approaches the circular waveguide loop32. The coupler separation d₁ of the coupler, i.e. the distance betweendata line 31 and loop 32, is typically between 0,2 and 0,4 μm. In theembodiment illustrated in FIG. 4, the directional couplers 33 and 34have electrical control inputs. A simplified top view of the coupler 33is shown in FIG. 4C. The control input, basically comprising two metalelectrodes 36 and 37, is electrical and has the capability of switchingthe coupler from the `bar-state` to the `cross-state`.

In order to allow a refresh by means of a counter propagating and/orco-propagating pump pulse, the memory loop has to be doped (at leastpartially) with Erbium ions, or with an appropriate amplifying medium asdescribed above.

Different kind of waveguide structures are known in the art. Typicalstructures are called stripe-loaded waveguide, buried channel waveguide,rib waveguide, embedded stripe waveguide, and ridge waveguide, just toname some of the well known structures. In the; present description andFigures only rib or ridge waveguide structures are shown for sake ofsimplicity. All other kinds of waveguide structures are as well suited.A typical example of a waveguide structure being suited for use inconnection with the present invention is given in FIG. 14. In thisFigure, a cross-sectional view of a stripe-loaded waveguide is shown.This structure is grown on a substrate 143 and comprises a first InPlayer 142 with lower refractive index n than the one of the InGaAsPwaveguide layer 141 on top of it. Lateral confinement of the light wavein said waveguide 141 is achieved by means of an InP ridge 140(`stripe`) situated on top of the waveguide layer 141. Due to thisloaded stripe, the light wave is confined as indicated by means of anellipse 144. In another embodiment layer 142 and ridge 140 may consistof SiO₂, whereas the waveguide 141 as such comprises silicon oxynitride,hereinafter denoted as SiON. Details on this and other structures andmaterials of waveguides and the fabrication techniques thereof are forexample given in a book titled "Guided-Wave Optoelectronics", edited byTh. Tamir, Springer-company, or in a book with title "Integrated Optics:Theory and Technology", R. G. Hunsperger, Second Edition, SpringerSeries in Optical Science Vol. 33, 1985. Typical waveguide materials areIII/V materials like AlGaAs or InGaAsP, glasses, silica, SiON, as wellas polymer films.

Further embodiments of the present invention are given in FIGS. 5A-5C.By means of these Figures, different configurations are described whichallow the refresh of a stored bit. Each of these systems comprises adata line 41, a directional waveguide coupler 43, and a memory loop 42.In the optical memory 40, shown in FIG. 5A, a passive directionalwaveguide coupler 44 with interaction length l is employed. The geometryand the distance between the pump line 45 and the memory loop 42 ischosen such that pad of the electro-magnetic wave which propagates insaid pump line 45, is automatically coupled into the loop 42 which isdoped with Erbium ions. In the next embodiment, illustrated in FIG. 5B,an optical memory 49 with separate pump line 46 is employed in order tofeed the refresh light pulse into the loop 42. This pump line 46 iscoupled by means of a Y-branch to the loop 42. The optical memory 50,which is illustrated in FIG. 5C, comprises an active semiconductorwaveguide region 48 being integrated into said memory loop 42. Thisactive region is employed to amplify the light pulse circulating in theloop 42 each time when it passes the active region 48, or each time whena refresh is required. By incorporating such a semiconductor opticalamplifier into the loop, a pump line is not required, but electricalaccess is necessary. Details on such semiconductor optical amplifiersare for example given in "Random Access Fiber Loop Optical Memory WithActive Switching and Amplifying Elements for Optical ATM Systems", R.Ludwig et al., proceedings ECOC (European conference on opticalcommunication), Paris Sep. 9-12, 1991, Vol. 1, MoC2-4, pp. 101-104.

The pump light may be delivered continuously, or in a pulsed manner. Thecontinuously delivered pump light has the advantage that there is noneed for synchronization of a pump pulse with the light pulsecirculating in a memory loop, if the respective directional waveguidecoupler is synchronized, respectively. The pump light could also besupplied selectively, either in a periodically clocked way, or with anarbitrary time pattern.

In order to reset an optical storage in accordance with the presentinvention, it is necessary either

1. to actively suppress the light pulse circulating in the memory loopby actively switching a pad of the loop to a higher absorption level,

2. to wait till a circulating light pulse disappeared due to losses(note: unpumped Er⁺ -ions show absorption), i.e. until the optical bitintensity drops below a certain decision level, or

3. to couple the light pulse out of the loop and to erase it. Activesuppression of the light pulse is possible in that the bandgap isactively changed, e.g. using the quantum confined Stark effect, or byactively increasing the losses in the waveguide. This can be donethermally, or by carrier injection, i.e. by modification of therefractive index n of the waveguide. Passive suppression is obtained bymeans of Er⁺ -ions in the ground state (unpumped ions) which causeabsorption in the waveguide. The absorption function could also beintegrated into one of the directional waveguide couplers, which wouldeventually reduce the number of contact metallizations needed. If onetakes the light pulse via a directional waveguide coupler out of theloop, the loop is reset and can be used for storage of the nextinformation. The light pulse which has been taken out of the loop bymeans of a directional waveguide coupler 53 can be erased in the dataline 51 as illustrated in FIG. 6. In this example, an active Y-branch isemployed, which allows to couple this pulse in an erase line 52 (dumpwaveguide). Information which is not to be erased, is coupled in the thedata output line 54.

Hereinabove, only single bit memory cells of different design andstructure have been addressed. In order to obtain an optical storageregister which is several bits wide, such single bit memory cells haveto be arranged in an appropriate manner. Depending on the degree ofintegration, the particular arrangement of cells, the organization ofwrite access, retrieval, refresh and reset, different optical memoriescan be obtained.

A first embodiment of a 4-bit optical random-access memory 60 isdescribed in connection with FIG. 7. This is the most simple arrangementconceivable. Each single memory cell comprises a data line 21, abirectional waveguide coupler 23, and a circular waveguide loop 22. Sucha 4-bit memory 60 has four individual input and output lines. Eachmemory cell can be viewed as separate and independent 1-bit memory. Thegreatest degree of flexibility is given if each memory cell is triggeredby its own control signal. A first degree of simplification (level ofintegration) can be achieved in that one control signal is used totrigger all cells. The latter approach allows storage of four parallelbits at once, i.e. with one control pulse applied to the directionalcouplers 23.

Another embodiment is illustrated in FIG. 8. In this Figure, a 4-bitoptical random-access memory 70 is shown which basically consists ofsingle memory cells, as described in connection with FIG. 4A. Eachmemory cell has its own data line 31, directional couplers 33 and 34,circular waveguide loop 32, and pump line 35. Again, different controlschemes with separate and/or common timing below the clock frequency areconceivable. These kind of 4-bit optical memories have four data inputsand outputs, as well as four pump signal inputs and outputs.

A further embodiment of the present invention is illustrated in FIG. 9.Shown is a 4-bit optical memory 80 having one common data line 81, fourmemory loops 82 through 85 and directional waveguide couplers 86-89. Afour bit sequence, for instance, which is fed via said data line 81 tothis memory 80, can be stored bit-by-bit in the memory loops 82-85. Thewhole sequence of four bits, or pad thereof, can be retrieved and fedback into the common data line 81.

Another 4-bit optical random-access memory 90 is shown in FIG. 10. Thismemory comprises separate data lines for each single bit memory cell anda common pump line 91 which is employed to refresh the data stored fromtime to time. This pump line 91 is optically coupled via directionalwaveguide couplers 92-95 to the memory loops and the loops aresequentially pumped. An equalization of the pumping level is achieved bydirectional couplers with different coupling fractions, i.e. In thepresent embodiment the coupling fraction of directional waveguidecoupler 95 might be higher than the one of coupler 92, to compensate thedepletion of the pump power.

In. FIG. 11, an embodiment is shown, which is a combination of thedevices shown in FIGS. 9 and 10. The layout of the pump line of the4-bit memories 90 and 100 allow some flexibility in pump-economics andusage of real-estate. Also it points to the usage of distributed pumpsources.

As the above described optical memories are essentially planar, more ofsuch planar waveguide circuitry can be stacked vertically either byfabrication or by mounting. This leads to higher storage densities andallows to perform more advanced all-optical processing and switching byemploying optical signals in the direction normal to the planarwaveguides. A cross-sectional view of an exemplary embodiment is givenin FIG. 13. In this Figure, two circular waveguide loops 121 and 122 areshown which are stacked on each other. On the right hand side both loopsare separated by a spacer 124. On the left hand side, a grating coupler123 is located between the lower loop 122 and the upper loop 121. Thegrating coupler 123 is employed to couple light out of the lower loopinto the upper one.

Depending on the environment in which the present optical memories areused, there is no need for directional couplers that can be activelyswitched from `bar-state` to `cross-state`. Simple passive directionalcouplers might be used in case where each light pulse which passes sucha coupler shall be coupled automatically into the loop. The arrangementof the directional couplers in said loops, the data coupler at one endand the pump coupler at the opposite end, is not mandatory. There aremany different arrangements conceivable that provide for a high degreeof flexibility when designing optical memories in accordance with thepresent invention.

What is claimed is:
 1. Random-access optical memory (20) with aplurality of memory cells each comprisinga data line (21), an activewaveguide coupler (23) which can be switched from a first state to asecond state, and a closed waveguide memory loop (22) for storing alight pulse (24) circulating therein,said data line (21) being opticallycoupled via said active waveguide coupler (23) to said memory loop (22)such that a light pulse (24) on said data line (21) is coupled into saidmemory loop (22) if the active waveguide coupler (23) is in the firststate, or passes the active waveguide coupler (23), without beingcoupled into said memory loop (22), if it is in the second state, thelength L of said memory loop (22) being such that the circulationfrequency of said light pulse (24) in said memory loop (22) is equal tothe clock frequency at which said optical memory (20) is designed to beoperated.
 2. The optical memory (20) of claim 1, wherein said memoryloop (22), waveguide coupler (23) and data line (21) are integrated onthe same substrate.
 3. The optical memory (20) of the claims 1 or 2,wherein said light pulse (24) represents one bit.
 4. The optical memory(20) of claim 1, wherein said waveguide coupler is a directionalwaveguide coupler (23) which can be electrically or optically addressedin order to switch from the first state (`bar-state`) to the secondstale (`cross-state`) and vice versa to selectively couple light pulsesfrom said data line (21) into said memory loop (22).
 5. The opticalmemory (20) of the claims 1 or 4, wherein said waveguide coupler (23) isused not only for coupling a light pulse into said memory loop (22), butalso for coupling a light pulse which circulates therein back into saiddata line (21).
 6. The optical memory (60, 70, 90; 80, 100) of any ofthe preceding claims, comprising at least two memory cells, the datalines (21, 31) of each memory cell being either independentlyaccessible, or the data lines of each memory cell being linked to eachother in a serial manner in order to form one common data line (81; 91;101).
 7. The optical memory (30; 40; 49; 50) of any of the precedingclaims, wherein means (44, 45; 46; 48) are provided for refresh of thelight pulse circulating in said memory loop (32; 42).
 8. The opticalmemory (30) of claim 7, wherein the refresh of the light pulsecirculating in said memory loop (32) is achieved by means of a pumplight pulse being fed via a pump line (35) into said memory loop (32)which is at least partially doped with Erbium ions, part of these Erbiumions being transferred from a ground-state into an upper-level state bysaid pump light pulse such that the light pulse circulating in saidmemory loop (32) is amplified by interaction with the Erbium-ions beingin an upper-level state when passing the section of the memory loop (32)which is partially doped with Erbium ions.
 9. The optical memory (30;40; 49) of claim 8, wherein said pump line (35; 45; 46) is coupled viaa)an active directional waveguide coupler (34), or b) a passivedirectional waveguide coupler (44), or c) a Y-branch,to-said memory loop(32; 42).
 10. The optical memory (50) of claim 7, wherein the refresh ofthe light pulse circulating in said memory loop (42) is achieved bymeans of a semiconductor amplifier section (48) being integrated intosaid memory loop (42).
 11. The optical memory of any of the precedingclaims, comprising at least two memory cells which are stacked on top ofeach other.
 12. The optical memory of any of the preceding claims,comprising III-V semiconductors, glasses, SiO₂ and SiON, or polymers.13. The optical memory of claim 12, wherein the substrate (143)comprises InP, the waveguide (141) comprises InGaAsP, and the ridge(140) formed on top of the waveguide (141) comprises InP.
 14. Theoptical memory of claim 12, wherein the substrate (143) comprises SiO₂,the waveguide (141) comprises SiON, and the ridge (140) formed on top ofthe waveguide (141) comprises SiO₂.
 15. The optical memory of claim 12,wherein the substrate (143) comprises glass, the waveguide (141)comprises a polymer, and the ridge (140) formed on top of the waveguide(141) comprises another polymer with lower refractive index than thewaveguide (141).
 16. A switch fabric for use in an optical datacommunication system comprising an optical memory in accordance with anyof the preceding claims.
 17. A computer in which part of the dataprocessing is done optically, comprising an optical memory in accordancewith any of the claims 1-16.